The invention relates to determining the dip of earth formations traversed by a borehole. It relates more particularly to a method of representing information relating to the dip of earth formations in the form of a polychromatic chart.
Dipmeter logs from boreholes have traditionally been used for defining the structural, stratigraphic, and sedimentological dip of earth formations. The design and operation of dipmeter logging tools, and the interpretation and graphic presentation thereof, are well known in this art, and will not repeated here. Reference is made to the book by Oberto Serra entitled Fundamentals of Well-Log Interpretation , Volume 1, "The Acquisition of Logging Data," (Elsevier, 1984). Chapter 19 of the Serra book, entitled "Dip Measurements (dipmeter logs)", at pages 269-302, is incorporated herein by reference thereto.
In conventional dipmeter logging, after the recorded data has been computer processed, results relating to the dip of the formations are usually presented in a graphic form which allows one to perceive the dip information visually. Among the possible visual presentations, the form which is used most often is called an "arrow-plot" and an example is shown in FIG. 1 (prior art). The X-axis of the arrow plot indicates dip angle, and the Y axis indicates depth along the borehole. Each data point on the arrow plot is represented by an oriented symbol having the appearance of a nail or tadpole. The head of the nail, by its (X,Y) coordinates, is a local indication of the dip angle as a function of depth, and the tip of the nail shows the direction of dip, with the upward direction indicating North. This type of presentation, often printed on paper, has long provided the hydrocarbon exploration industry an easy-to-read two-dimensional chart of three-dimensional logging information: depth, dip angle, and dip direction.
Referring again to FIG. 1 (prior art), some of the nails are coded black while some are white, to indicate the technical quality of the measurement which produced the dipmeter data. This additional presentation convention has been used in the art to distinguish "average" to "good" measurements (black coded nail heads, e.g. point 1 on FIG. 1) from poor measurements (white coded nail heads, e.g. point 2 on FIG. 1). It will readily be seen that in the use of such a coding scheme, the human eye is naturally drawn to the black nail heads and automatically tends to pay less attention to the white nail heads.
However, when all of the measurements are of very good quality, giving rise to a large density of black nail heads, the human eye is considerably less capable of perceiving and grasping the global information relating to dip direction. In addition, some nail heads may mask the tips of adjacent nails, and obscure the global or total information content of the log.
It is therefore an object of the invention to provide a method of presentation of log information of the above kind, which remedies the above-mentioned drawbacks.
It is additionally an object of the invention to provide a method of presentation of 3-dimensional well logging information in a 2-dimensional plot which avoids obscuring the total information content, while enhancing the visual perception of the most useful information by the human eye.